This invention relates to a pressure-control unit for a vehicular hydraulic braking system including a pressure-reducing valve and a pressure-limiting valve arranged coaxially therewith, with an intermediate piston which is acted upon by the output pressure between the pressure-reducing valve and the pressure-limiting valve.
In a known pressure-control unit, if both brake circuits are intact, the intermediate piston causes the output pressure of the pressure-limiting valve to follow the output pressure of the pressure-reducing valve above the changeover point. This is achieved due to the fact that in the event of a differential pressure the intermediate piston becomes displaced and thereby influences the closure members of the two valves in such a way that the output pressures approximate each other, while their magnitude is determined by the pressure-reducing valve.
Because of the frictional resistance of the seals of the intermediate piston, a pressure differential is required for its displacement which differential is determined by the cross-sectional area of the intermediate piston and its frictional resistance. This pressure differential should be as small as possible in order to render the two output pressures as even as possible. This results in the requirement for the intermediate piston to have a diameter as large as possible because the ratio between the cross-sectional area and the frictional resistance becomes more favorable as the diameter increases, so that the intermediate piston becomes displaced at a small pressure differential, i.e., the hysteresis becomes smaller.
When dimensioning the intermediate piston, it is also necessary to consider the case that the brake circuit controlled by the pressure-reducing valve fails. In this instance, the output pressure of the pressure-limiting valve is only counteracted by the control force of the pressure-reducing valve, which results in a real limitation of the output pressure of the pressure-limiting valve. In this process, the magnitude of the limited output pressure is dependent on the magnitude of the control force and the cross-sectional area of the intermediate piston. Since the desired output pressure is as high as possible in this case, the cross-sectional area and thus the diameter of the intermediate piston must be as small as possible.
Hence, there are two directly contradicting requirements with respect of the dimensions of the intermediate piston; as a rule, the middle course is chosen. This middle course is, however, not fully satisfactory.